I'm actually running a case, using a kind of chtMultiRegionFOAM, just a bit modified (orthotropic definition of material). I don't understand the mean of convergence we have, and the meaning of the timeStep in controlDict.
I explain : If I put 0.0001 for a timeStep, it's EXACTLY the same, when I post process, as if I put 10000 for a timeStep. It's juste ONE timeStep.
So, in my case, I've got two solids and a fluid. The residuals of the fluid converge very quikly (see graph). But the temperature of my solid (Box2) is VERY VERY nit (4 hours with 4 processors => 8000 timeSteps) (See graph).
And I don't understrand physically why....

EDIT : Ok, I found a solution : Put T relaxation to 1.0 (It was 0.7 before).
32 times faster.